Book the two seats in one transaction, that will normally have them placed together.

do you mean at the same time? thank you

Yes, at the same time, search for 2 seats and book them, the reservation computer will, as long as it is possible, place them together.

Thank you!

And how can i book a reservation for us when we left our country of residence and can use the interrail tickets together? do i have to continue the trip i do before? Or can i connect to my pass to the other trip?

Reservations are separate from the pass. You can have your passes on different phones or on the same one, but that doesn’t make a difference for the reservations. But it's usually better to have them on different phones, so you can also travel separately if needed.

As to booking reservations in general:

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new topic, and you will get advice.

Please note that Interrail charges an extra fee of 2 EUR per person and ticket in addition to the fee for the seat reservation.

First you should read this page:

https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started

and then this thread on reservations for specific countries

https://community.eurail.com/train-connections-reservations-47

Thank you! You have helped me a lot! :)

But i think i will change my plans.. Is it possible to change my 1 month ticket to a 5 days in a month ticket, like my friends have, without losing my reservations?

The reservations will remain.

If the conditions for your pass allows, you can make an exchange of your pass. The conditions should be visible in your Interrail account and on the invoice. 

There is a fee of 15 EUR for an exchange. You first have to buy the new pass and then send both the old and the new pass numbers to Customer Support. They wil then refund the price of the first pass less 15 EUR.

You'll find all information you need in your Interrail account under "refund and exchange"

The pass you currently have must be deactivated in order to make the exchange.